The invention concerns a four-nozzle plasma generator for forming an activated jet.
This generator can be used in particular for surface treatment processes (sterilisation, cleaning, etching, modifying, depositing coatings and films), of disperse and solid state materials, and for obtaining chemical products in the field of electronics, the automotive industry, the food industry, medicine, chemistry, the manufacture of machines and tools, etc.
A four-nozzle plasma generator is known which comprises two anode and electrode chambers connected to DC power sources and generates four plasma jets whose shape and trajectory is dictated by an external magnetic field system, such that the plasma jets converge to form a single plasma stream of lowered temperature at the central zone, into which the chemical products and/or materials to be processed are introduced, than in the peripheral zones. This device is described in the document entitled &lt;&lt;Basis for implementation of the method for dynamic plasma treatment of the surface of a solid body&gt;&gt;P.P. Koulik et al. &lt;&lt;Plasmochimie 87", Moscow 1987, part 2, pp. 58 to 96.
The construction of the electrode chambers (anode and cathode) is described in the document entitled &lt;&lt;Twin jet plasmatron&gt;&gt;, I. I. Genbaiev, V.S. Enguelsht, Frounze 1983.
The advantage of this generator of the prior art is due firstly to the specific configuration of the plasma streams which form a plasma funnel, enabling efficient introduction and processing of different products. Secondly, the electric current passing through the plasma jet, heating it and activating it with minimum losses, in view of the absence of cooled walls, means that the device offers high performance in output.
This device can be used efficiently for surface sterilisation, cleaning, modifying, etching and depositing coatings and films. Practical application of this reveals the following disadvantages however:
1. The generation of jets and streams of plasma are accompanied by toroidal vortices. The resulting hot gas flow heats the parts of the electrode chambers and the fixing and supply elements on the one hand and causes substantial heat losses by thus reducing the generator efficiency. On the other hand, in certain cases, the degree of turbulence of the plasma stream is increased and there is a loss of products introduced into the central zone of the stream, generating harmful secondary effects in terms of the service life of the generator, as these products precipitate on the surface of the electrode chambers and of the fixing and supply elements. Plasma radiation, which is particularly high when chemical products are introduced into the plasma stream, is also a cause of superfluous heating of the various parts of the generator exposed to this radiation. PA1 2. Once the plasma stream has finished heating and activating the products introduced into it, its role then ends and its presence in the periphery of the activated product stream is no longer required. If the activated product is a gas (which is the case in many applications, in particular surface cleaning, etching, films depositing, surface modification), the presence of the original plasma in the periphery of the activated stream starts to hinder the efficiency of the surface treatment. In fact, the plasma, which is still hot, heats the treated surface, and this is generally to be avoided. In addition, the plasma becomes just a passive component on the surface. Not involved in interaction with the surface, the only acting particles being those which the plasma has activated and which reach the surface by diffusion. The residual plasma is an obstacle to this diffusion, proving an even greater hindrance as the efficient section of the corrected particles forming the plasma is, on average, of an order larger than that of the neutral particles (activated) and hence the presence of these particles substantially reduces the coefficient of diffusion of the activated particles, and thus the diffusion stream, and finally the efficiency of the treatment.
The result of the simultaneous effect of the convection and radiation streams is, ultimately, that the service life of the generator is reduced due to superfluous temperature rise of its parts and the formation of layers of precipitates of low thermal conductivity, making cooling of these parts difficult. PA2 In addition, from time to time, the layers of precipitate break down and are carried away by the gas vortices, soiling the treated surfaces and the plasma stream itself, making the composition of the latter virtually uncontrollable.